Display actual nested trees in the Admin UI

[janus-reith]

For Pages it would be great to have a collapsible page tree.
The nestedDocs plugin probably already covers the database facing parts for three structures, so probably it's just a matter of visual representaton.
I think this could be a reasonable core addition since its a typical feature of many CMS Admin panels.

[sannajammeh]

I second this!

One of the reasons we can't yet more many projects over to Payload is the lack of nested folders, especially for Media objects.

[GermanJablo]

Closing because it is included in #4190
This feature is a WIP, so stay tuned!